Hi Puma, good question
I suspect what you’re actually seeing is the effect of flap's on your pitching moment.
As you’d expect on a low wing aircraft adding flaps causes the nose to pitch down.
(generally high-wing aircraft want to pitch up with flaps)
So you’ll need to account for that as you flight test.
The flaps initially increase lift, drag, and down moment; and at 100% the effect is dominated by drag and moment.
One simple way to isolate flap effects is to use the autopilot to maintain your altitude.
Allow for enough time after reconfiguring to let the forces stabilize.
It’s tricky to get a power setting low enough to extend flaps clean, yet high enough to hold an alt with flaps…because the aircraft will transition into slow flight. It’ll want more power.
But to observe the flap effects in isolation both power and alt must remain constant.
Another way to watch the flap effects empirically is to use a tool like AFSD.